A magnetotelluric transect of the Wairarapa region, New Zealand

Transcription

1 McLoughlin New Zealand et Journal al. MT of transect, Geology Wairarapa & Geophysics, 00, Vol : /0/0 057 $7.00/0 The Royal Society of New Zealand A magnetotelluric transect of the Wairarapa region, New Zealand CHRIS McLOUGHLIN Institute of Geophysics Victoria University of Wellington P.O. Box 600 Wellington, New Zealand MALCOLM INGHAM School of Chemical & Physical Sciences Victoria University of Wellington KATHY WHALER Department of Geology & Geophysics University of Edinburgh West Mains Road Edinburgh EH9 3JW, United Kingdom DON McKNIGHT Institute of Geological & Nuclear Sciences P.O. Box 368 Lower Hutt, New Zealand Abstract Magnetotelluric (MT) soundings have been made at 4 sites along a traverse of the Wairarapa region at the southwest end of the active Hikurangi margin. Joint inversion of direct current resistivity soundings and the MT data indicates that the MT data are unaffected by static-shift. Analysis of the dimensionality of the data suggests that the transverse electric (TE) mode data (derived from an electric field parallel to the strike of the margin) may be distorted by 3D effects arising from the presence of Cook Strait and the non-continuity of structure along the margin. D inversion of the more robust transverse magnetic (TM) mode responses has therefore been used to derive a model of electrical resistivity structure beneath the transect. This shows the existence of two depressions containing lowresistivity sediments separated by a more resistive region which is inferred to correlate with a spur of Mesozoic greywacke extending north from the Aorangi Mountains. The same features were previously identified from gravity data. The thickness of sediments adjacent to the Wairarapa Fault is of the order of 3 4 km, whereas that in the western arm of the Wairarapa trough is c. 3 km. The resistivity structure of the southwest end of the Hikurangi margin is more complex than the structure determined farther north along the margin. Keywords magnetotellurics; Hikurangi margin; resistivity G0003; published June 00 Received 6 January 00; accepted 9 October 00 INTRODUCTION Ingham et al. (00) have presented the results of a magnetotelluric (MT) study of the southern portion of the onshore part of the Hikurangi margin (Fig. ). The Hikurangi margin is a region of ongoing deformation (Walcott 978) that lies to the northwest of the Hikurangi Trench along which the Pacific plate is subducted beneath the Australian plate off the east coast of the North Island of New Zealand. Forward modelling of MT data along a transect across the margin (Fig., BB ) identified a thin zone of conductance (conductivity-thickness product) not less than 400 S at the depth of the interface between the subducted and overriding plates. This is most probably related to the presence of subducted sediments and correlates with the identification of a low-velocity zone at the plate interface (Reading 996; Chadwick 997). Similar features have been noted in other subduction systems (e.g., Kurtz et al. 986, 990; Wannamaker et al. 989a, b; Fuji-ta et al. 997). Ingham et al. (00) also reported a perpendicular MT transect (Fig., AA ) running along the length of the margin, which penetrated into the Wairarapa region at its southern end. Structure along much of the length of this transect was interpreted to be D, and showed continuity of the conductive Tertiary sediments into the Wairarapa region. A close correlation was also observed with seismic results (Chadwick 997). At the southern end of the transect, an increase in resistivity was observed that was inferred to be related to the exposed greywacke of the Aorangi Mountains (Fig. ). The Wairarapa region is located in the southern part of the Hikurangi margin along the east coast of the North Island of New Zealand (Fig. ). The Wairarapa valley (Fig. ) is bounded on its west side by the Wairarapa Fault, which separates Mesozoic greywacke of the main axial ranges from Quaternary river gravels and Cenozoic sediments that lie to the east. One of several strike-slip faults that form the North Island Dextral Fault Belt (Beanland 995), the Wairarapa Fault is estimated to have a long-term average slip rate of mm/yr (Grapes 99; Beanland 995). The basement in this region consists of faulted and folded Mesozoic greywacke and argillite (Kingma 967). Only a few geophysical investigations have been made to determine the deeper geological structure of the region. The nature and dip of the Wairarapa Fault at various locations has been studied using gravity measurements by Gresham (967), Hicks & Woodward (978), Bowler (998), and McClymont (000). Heine (964), Gibbs (969), and Hicks & Woodward (978) have presented more widespread gravity data from across the Wairarapa region. These studies indicate that the dip of the Wairarapa Fault varies along its length and in the southern part of the region is believed to be reverse. Most detail of the regional structure is provided by the results of Hicks & Woodward (978), who used both D and 3D modelling. They suggested that the Wairarapa

2 58 New Zealand Journal of Geology and Geophysics, 00, Vol. Fig. Generalised geology of the Hikurangi margin in the southern part of the North Island of New Zealand. AA and BB are the MT transects discussed by Ingham et al. (00). The box encloses the area shown in Fig.. valley (or trough) is a fault-angle depression which is split in its southern part by a spur of Mesozoic greywacke extending north from the Aorangi Mountains (Fig. ). Hicks & Woodward (978) put the thickness of sediments adjacent to the Wairarapa Fault as up to 3 km, with a somewhat lesser thickness in the eastern part of the trough. Cape (989) and Cape et al. (990) reported seismic reflection data from two parts of Wairarapa. Beneath an c. 0 km line just to the south of Masterton, the basement surface was found to be undulating and to occur at a depth of between and km. Farther to the south, a much shorter seismic line could not detect basement but showed the sediments to dip at c. to the northwest. Presented in this paper are the results of a second MT transect across the Hikurangi margin located some 00 km southwest of that reported by Ingham et al. (00) and crossing the Wairarapa region. The main aim of the study was to determine if the conductive zone detected to the north was also present farther south along the strike of the Hikurangi margin. Given the lack of detailed knowledge of the crustal structure in the region, a secondary aim was to improve determination of crustal structure through the derivation of an electrical cross-section of the Wairarapa region. MAGNETOTELLURIC DATA Magnetotelluric measurements were made at 4 locations (Fig. ). Data at 0 of the sites were collected over the summer of 998/99, while 4 sites were occupied in conjunction with the study reported by Ingham et al. (00). Two of the sites (MGN, KAI) lie to the west of the Wairarapa Fault on the Mesozoic greywacke rocks that make up the main axial ranges of the North Island. A further site, PIGB,

3 McLoughlin et al. MT transect, Wairarapa 59 Fig. Locations of MT sites, and features and places mentioned in the text, in the Wairarapa region. is located immediately adjacent to the fault on the northwest side. Five of the sites (FEA, LWD, TAI, MARA, ONGA) lie in the central part of the Wairarapa valley, while five other sites (MART, MAR, PLAV, HIKA, HIN) are located to the east on the Tertiary sediments of the outer part of the Hikurangi forearc. The site, PAHA, is located on outcropping greywacke in the coastal ranges at the eastern end of the transect. The average intersite spacing along the transect is 6 km and is able to provide good resolution of lateral variations in electrical conductivity. This is particularly true in the central part of the transect where sites are more closely spaced. Sites with three-letter acronyms were recorded with the Victoria University MT system, which has a nominal period range of s. Data at sites with four-letter acronyms were collected using a SPAM Mark III system (Ritter et al. 998) and have a nominal period range of s. At all sites, data were recorded for between 4 and 48 h. The compatibility of data recorded by the two separate systems has previously been demonstrated by Ingham et al. (00). The recorded data were processed to obtain robust estimates of the impedance tensor relating the horizontal electric and magnetic field variations. However, as the Wairarapa region is intensively farmed, and remote referencing of MT sites was not available, cultural noise from a variety of sources (e.g., power lines, electric fences) both restricts the bandwidth of the useable data and causes scatter in the data, particularly in the phase, at some sites. Examples of apparent resistivity and phase curves rotated to an orientation that corresponds to the gross tectonic strike of the region (N35 ) are shown in Fig. 3, and pseudosections of apparent resistivity and phase in these orientations for all sites are shown in Fig. 4. The main features of the data pseudosections (Fig. 4) appear broadly in both the transverse electric (TE) and transverse magnetic (TM) mode. At the northwest end of the transect, high apparent resistivity and generally low phase values are observed at sites located on greywacke. In contrast, much lower apparent resistivity values are observed at those sites at the southeast end of the transect, on the outer part of the Hikurangi forearc (e.g., PLAV, Fig. 3). In the central part of the transect, the apparent resistivity values are generally low but higher values exist (and are particularly clear in the TM mode pseudosection) between ONGA and MAR. Associated with the central part of the transect is a rather complex series of phase variations. At short periods, relatively high phases appear to delineate two basins of higher conductivity. At longer periods, the TE phase in this region is significantly lower (e.g., TAI, Fig. 3) and becomes very close to 0 at periods around 0 s. The two regions of high phase values observed at short period are separated by a region in which phase values are lower and which spatially correlates with the enhanced apparent resistivity between ONGA and MAR. Significant lateral changes in both apparent resistivity and phase persist to the longest period, suggesting that the crustal structure within the overlying Australian plate may be sufficiently complex as to mask any possibility of imaging the subducted Pacific plate. INVESTIGATION OF STATIC-SHIFT A perennial problem in magnetotelluric sounding is the possibility of static-shift of the apparent resistivity data (Jones 988). Static-shift is thought to be caused by the existence of small near-surface resistivity inhomogeneities that result in a frequency independent distortion of the ambient electric field. The effect on apparent resistivity data is manifested as an upward (or downward) shift of the entire curve without change of shape. data are unaffected. Without correction for static-shift, interpretation and modelling of MT data can lead to significant errors in the derived electrical resistivity structure. Although static-shift may sometimes be clearly evident in recorded data, this is not always the case, and a suitable means of checking for its presence is necessary. To correct for static-shift in the apparent resistivity data from the northern transect of the Hikurangi margin, Ingham et al. (00) shifted the apparent resistivity curves so that the short period apparent resistivity values matched the deepest resistivity derived from direct current (dc) resistivity

4 60 New Zealand Journal of Geology and Geophysics, 00, Vol. TAI PLAV Fig. 3 Examples of MT data. Data are presented in a co-ordinate system rotated to N35 E, the gross tectonic strike of the region. Circles correspond to data with the electric field oriented parallel to this direction (TE mode), crosses to data where the electric field is oriented perpendicular to this direction (TM mode). Apparent resistivity (Rhoa) is in Ωm, period (T) in s, and phase in degrees. Uncertainties shown are the larger of standard deviation or 0. in the log of apparent resistivity and 3 in phase.

5 McLoughlin et al. MT transect, Wairarapa 6 Log(Apparent resistivity) - TE L og(apparent resistivity) - TM L NW MARA SE NW MARA SE FEA TAI MART PLAV FEA TAI MART PLAV MGN KAI P IGB LWD ONGA MAR HIKA HIN PAHA MGN KAI P IGB LWD ONGA MAR HIKA HIN PAHA TE - TM km km Fig. 4 Data pseudosections of log(apparent resistivity) and phase. Ordinate is the period in s. TE refers to an orientation of the electric field of N35 E, TM to an orientation of N55 W.

6 6 New Zealand Journal of Geology and Geophysics, 00, Vol. soundings at each site. This procedure was successful because at many of the sites the ambient near-surface resistivity was sufficiently low that there was an overlap in the depths sampled by the dc and MT soundings. Where this overlap did not exist, uniformity of the resistivity structure with depth meant that the estimate of resistivity derived from the dc sounding was still applicable at the depths sampled by the short-period MT data. Direct current (dc) resistivity soundings, using the Schlumberger array with a maximum current electrode halfspacing of 80 m, have been made at all the MT sites on the present transect except for MAR and PAHA. However, attempts to use the same procedure as adopted by Ingham et al. (00) show that this is not valid at many of the sites where the near-surface resistivity is relatively high and the depth ranges of the dc and MT data do not overlap. Nor is there uniformity of resistivity with depth. For example, at LWD the dc resistivity data suggest that at a few 0 s of metres depth the ambient resistivity is c. 00 Wm. The shortperiod MT data indicate an apparent resistivity of c. 0 Wm. If the MT curve is shifted so that the short-period apparent resistivity matches the value derived from the dc data, then simple D modelling of the resulting MT curves produces a resistivity structure which is not consistent with the known geological structure of the site. Clearly a different method for assessing and correcting static-shift must be applied. An alternative procedure that uses the dc data to correct for static-shift involves several steps. Initially, the dc resistivity sounding from each site is modelled in terms of a layered resistivity structure. This layered structure is then used in a D forward modelling scheme to simulate MT data at shorter periods (higher frequencies) than those covered by the actual MT data at the site. The high-frequency simulated MT data are plotted with the recorded MT data. Visual continuity between simulated and measured data for both the apparent resistivity and the phase data gives a first indication that there is no reason to assume that there is any static-shift in the MT data. A mismatch between the two datasets in apparent resistivity, but not in phase, indicates the presence of static-shift in the apparent resistivity data and allows the magnitude of the shift to be estimated. A mismatch in both apparent resistivity and phase data is taken to indicate that significant resistivity structure exists at depths which fall between the deepest sampled by the dc data and the shallowest sampled by the MT data. With the exception of one site (MART), the simulated MT data have been matched to apparent resistivity and phase data derived from the determinant impedance (Ranganayaki 984). The determinant impedance is one of a number of rotationally invariant parameters that can be formed from the MT impedance tensor and can be regarded as being indicative of the variation of resistivity with depth beneath a site when D and 3D effects are not dominant. The absence of any offset between the shortest period TE and TM data at any of the sites means that matching the simulated data with the determinant data also provides a match with the TE and TM data. As a result of cultural noise, at MART, the TM responses at short periods are very scattered. This has the effect of introducing scatter into the determinant responses. For this reason, at MART, the TE responses have been used to match the simulated responses derived from the dc data. At periods > s, the TE and TM responses are very similar, and it is therefore inferred that the use of the TE mode rather than the determinant data makes little difference to the result. When this procedure is applied to the present data, it is found that for all the sites the simulated high-frequency apparent resistivity and phase data derived from the dc resistivity sounding are both visually continuous with the actual MT data. There is therefore no initial indication that static-shift corrections of the apparent resistivity curves are necessary. An example of the match between simulated and actual MT data at site FEA is shown in Fig. 5. Support for this inference is obtained by using the resulting expanded dataset (i.e., simulated plus actual MT data) for each site as a basis for D inversion. The absence in the inversion of any major discontinuity in resistivity structure associated with the matching of the simulated and actual MT data confirms the visual matches of both the apparent resistivity and phase data. The inversion procedure that has been used is the Occam D inversion of Constable et al. (987), which produces the smoothest variation of resistivity with depth (i.e., it only includes structure demanded by the data). The derived resistivity structure at FEA and the fit that it gives to the expanded dataset are also shown in Fig. 5. The decrease in resistivity between 0 and 00 m depth derives entirely from the dc data, and the significant resistivity structure beneath km depth results from the actual MT data. No significant additional structure is produced in the inversion by the matching of the simulated and actual data. D INVERSION OF THE DATA The D inversions of the expanded dataset for each site may also be used to give a first indication of the variation in electrical structure along the transect. A compilation into a D section of the results of the Occam inversions for of the 4 sites is shown in Fig. 6. Due to high levels of cultural noise, data from HIKA and PAHA are of limited bandwidth and have been omitted from the D inversion and from further analysis. The derived structure shows the same basic features as were originally deduced from inspection of the data pseudosections. At the southeast end of the transect, the Tertiary sediments of the onshore part of the outer Hikurangi forearc have a resistivity of c. 0 Wm. Beneath the Wairarapa valley, another area of similarly low resistivity occurs and extends to several kilometres depth. These two low-resistivity regions are separated by a more resistive block which extends from depth almost to the surface. The greywacke main ranges in the western part of the transect have high resistivity, but the relatively coarse site spacing does not image a sharp boundary between the conductive sediments and the more resistive greywacke. A failure of the D smooth inversions is the inability to clearly delineate the depth extent of the conductive sediments. Such information is likely to be better resolved by D inversion of the data, and is considered in the next section. DIMENSIONALITY OF THE DATA, REGIONAL STRIKE ORIENTATION, AND D INVERSION D inversion using rotationally invariant determinant apparent resistivity and phase data can be argued to be an appropriate means of obtaining a first indication of the resistivity structure. Considered use of D inversion and

7 Depth (km) Log {Depth (m)} 0 3 McLoughlin et al. MT transect, Wairarapa 63 FEA - matching of simulated and measured MT data -D inversion of simulated and measured MT data Log {Rho (Ohm.m)} Fig. 5 Matching of simulated MT data derived from dc resistivity sounding with measured MT data. The resistivity-depth profile results from joint D inversion of the apparent resistivity and phase data shown on the left. The solid line shows the fit of this inversion to the data. Errors in the simulated MT data are set at 0.05 in log(ρ a ) and 3 in phase. Fig. 6 Resistivity structure resulting from D joint inversion of dc and MT data. Contours are of log(ρ). Vertical exaggeration. NW 0 Mgn Pigb Lwd Mara Kai Fea Tai H Onga Mart Mar Plav in SE Distance (km) modelling, however, requires not only justification that it is appropriate to treat the data in this manner, but also identification of the electrical strike orientation which the data reflect. Although the present MT transect is perpendicular to the gross regional tectonic strike, there are several features which might impose 3D characteristics upon the data. The major such influence is likely to be the presence of Cook Strait to the southwest, where the electrical conductivity contrast between the highly conducting sea water and more resistive landmass is oriented approximately perpendicular to the regional tectonic structure (see Fig. ). To a lesser extent, the finite length of the Wairarapa valley along the strike of the regional trend, and the presence of outcropping greywacke in the Aorangi Mountains, may also influence the data. Various techniques (e.g., Groom & Bailey 989; Groom & Bahr 99; Lilley 998a, b) exist to analyse the dimensionality of MT data and to reduce the data to the most appropriate D form by removing the effects of galvanic distortion. Ingham (996) used the technique of Groom & Bailey (989) to analyse distortions and to determine the regional electrical strike direction for MT measurements on a transect of the South Island of New Zealand. For the analysis of data from the Hikurangi margin to the north of the present transect, Ingham et al. (00) used the decomposition technique of Lilley (998a, b). A comparison

8 New Zealand Journal of Geology and Geophysics, 00, Vol. of these techniques for data from the Alpine Fault was presented by Ingham & Brown (998). Lilley s decomposition is based on the representation of the impedance tensor by a Mohr s circle and acts separately on the real and quadrature parts of the impedance tensor to recover, for each, an estimate of the regional strike direction as a function of period. This direction may vary with period either because data are in fact D in character, in which case there is no clear definition of a strike direction, or because 3D structure imposes a genuine variation of strike orientation with period. The present MT data are in fact quite markedly D in nature at short period, and therefore it is a reasonable assumption that the best indication of the behaviour of the regional electrical strike will be obtained by decomposition of the long period impedance estimates. It can additionally be noted from Fig. 4 that phase values at periods > s are generally low (c. 0 ) in both the TE and TM modes. This means that the quadrature parts of the impedance tensor elements are considerably smaller than the respective real parts. A consequence of this is that, for even quite moderate levels of uncertainty and scatter in the impedance estimates, a strike direction obtained from decomposition of the quadrature part of the impedance tensor is likely to be much less well constrained than one determined from the real part. In the present case, decomposition of the real part of the impedance tensor at periods > s is therefore used to determine estimates of the regional strike of an assumed D electrical conductivity structure. The results of such a decomposition using the method of Lilley (998a) are illustrated in Fig. 7. Orientations of the derived strike direction are plotted for each of sites for impedance estimates at periods > s. For clarity, the determined directions are shown only for four periods per decade. The strike orientations recovered by the decomposition process are ambiguous by 90, thus if the data are D with a regional electrical strike which is compatible with the gross tectonic strike of the region, then the calculated orientations should be close to either N35 E or N55 W. It is apparent from Fig. 7 that this is indeed the case for the majority of sites. Significant deviations from these two orientations occur only at KAI, ONGA, and MAR. At both ONGA and MAR there is no predominant strike orientation. At these two sites, the apparent resistivity and phase data at long periods are similar in both TE and TM modes, and it seems reasonable to assume that the lack of constraint on strike orientation results from being D, rather than a true variation of regional strike with period. However, it is possible that at MAR the variation in strike direction may reflect the geological boundary between Quaternary/Tertiary/ Cretaceous rocks to the northeast and Jurassic/Triassic greywacke that makes up the Aorangi Mountains to the southwest. At KAI, a reasonably well determined strike direction is obtained which corresponds to neither N35 E nor N55 W. KAI, in a valley in the main ranges, is situated on Quaternary river gravels overlying Mesozoic greywacke. The apparent resistivity in both TE and TM modes increases more or less uniformly with period (Fig. 4). A possible explanation of the strike orientation at KAI is that, given the resistive nature of the subsurface, the long period skin-depth is sufficient that the site is sensitive to the proximity and orientation of Cook Strait. For example, at a period of 0 s for a resistivity of 000 Ωm (a typical value for the resistivity of greywacke) MGN KAI FEA LWD TAI N PIGB MARA PLAV N MAR MART HIN ONGA Fig. 7 Long-period strike orientations determined for the real part of the impedance tensor by the technique of Lilley (998a). the skin-depth is c. 50 km. This is the same order of magnitude as the distance of KAI from both the northwest and southern coasts bordering Cook Strait, and indicates that it is highly likely that the data at KAI are influenced by the 3D nature of the land/sea boundary. At KAI, the D assumption must therefore be treated with considerable caution. In contrast, two other sites at the northwest end of the transect are also situated on greywacke (MGN and PIGB) but return regional strike directions that are compatible with

9 Depth (km) McLoughlin et al. MT transect, Wairarapa 65 Fig. 8 Resistivity structure resulting from D inversion of TM mode MT data. Contours are of log(ρ). Vertical exaggeration. NW 0 Mgn Pigb Lwd Mara Kai Fea Tai H Onga Mart Mar Plav in.5 SE Distance (km) the tectonic strike. This can be explained by the fact that both sites are located in close proximity (i.e., much closer than one skin-depth) to significant lateral electrical conductivity contrasts which closely follow the regional trend. Data at MGN, however, extend only to c. 3 s period. If the period range were to be extended, it is possible that the 3D nature of the coastline would in fact start to have a significant effect. The influence of 3D structure imposed on data at periods longer than 0 s by the presence of Cook Strait presents a strong reason why any attempt to resolve the presence of a conductive layer at the plate interface seems unlikely to be successful along this transect. On the transect 00 km to the north, Ingham et al. (00) found indications of such a conductor through D forward modelling of data in the period range s. On the present transect, even at sites on the conductive outer forearc, the skin-depth at these periods is a significant fraction of the distance of the site from Cook Strait. In such a situation, the consequent 3D effects in the data at long periods mean that D forward modelling is unlikely to be able to resolve any thin interplate conductor. The main focus of the present study therefore becomes the resolution of variations in crustal electrical resistivity structure across the Wairarapa region. The results of the decomposition support the assumption that the data are largely D, and that the regional electrical strike is close to the tectonic strike of N35 E. However, as discussed above, a degree of caution is required as the possible influences of Cook Strait, the Aorangi Mountains, and finite along-strike length of some tectonic features may in fact produce effects that, with the ambiguity of 90 in strike determination, cannot be resolved. Nevertheless, the decomposition yields estimates of the regional (i.e., TE and TM) apparent resistivity and phase curves corresponding to the calculated strike orientations. Adjacent to the Alpine Fault, Ingham & Brown (998) found very little difference between estimates of the regional responses obtained by decomposition and the original data rotated to the same orientation. Comparison of the original estimates as plotted in Fig. 4 with the regional apparent resistivities and phases obtained from the decomposition shows the same to be true in the present case. In the absence of significant differences between the two sets of response estimates, the original apparent resistivity and phase data have been used to derive the D resistivity structure beneath the transect. In a fully D situation, joint inversion of the TE and TM mode responses would be preferable and expected to give the most accurate recovery of electrical structure. However, the TM mode apparent resistivity and phase data, being derived from an electric field perpendicular to the main strike direction, are less likely to be affected by 3D effects arising from a degree of non-continuity of structure along the strike direction. The TM data therefore have been used as the basis for D inversion using the RRI code of Smith & Booker (99). The greater robustness of the TM mode data to 3D effects has been discussed previously by Wannamaker (999). In the present case, separate inversion of the TE mode data is discussed below and has been used as a first order check on the derived structure. A D electrical resistivity model derived from the TM data using the robust inversion option in the code of Smith & Booker (99) is shown in Fig. 8. The resistivity structure is similar to that derived by the compilation of the D inversions (Fig. 6), but several differences do exist. Below c. km in depth, a much sharper lateral gradient in resistivity occurs between FEA and PIGB across the Wairarapa Fault, and the resistivity low beneath FEA, LWD, and TAI is intensified. The resistivity high between this region and the low corresponding to the onshore part of the forearc is better defined and extends roughly from ONGA to MART. The lack of resolution of the base of the Tertiary sediments in the D compilation is overcome with a sharp rise in resistivity below 3 km depth to the southeast of MAR. In general, the derived resistivity structure fits the TM mode data, especially the apparent resistivity data, extremely well. This is illustrated in Fig. 9, which shows the fit of the model responses calculated from the resistivity structure of Fig. 8 to the apparent resistivity and phase data at the sites. The input TM mode data are a subset of the complete TM dataset and cover the period range s. The main misfit in the phase data is that the model does not reproduce the complete range of phase variation that occurs in the central part of the transect. Scatter in the short-period phase estimates contributes to the degradation of the fit at some sites. The main features and trends in the phase data, namely low phase at the northwest end of the transect and two regions of higher phase separated by lower phase beneath MART, are reproduced, however. In an attempt to check on the consistency of the inversion, the TE mode apparent resistivity and phase data have been inverted separately from the TM mode data. The resulting resistivity structure shows the same main features that are derived from the TM data but the shapes of these are modified somewhat. The most significant differences are a thickening of the low-resistivity regions in both the outer forearc at the southeast end of the transect and in the western

10 66 New Zealand Journal of Geology and Geophysics, 00, Vol. MGN KAI PIGB FEA LWD TAI MARA ONGA

11 McLoughlin et al. MT transect, Wairarapa MART MAR PLAV HIN Fig. 9 Fit of the apparent resistivity and phase responses of the model shown in Fig. 8 to the TM data from each site.

12 68 New Zealand Journal of Geology and Geophysics, 00, Vol. arm of the Wairarapa trough adjacent to the Wairarapa Fault. The ridge of resistive material separating the two branches of the trough is also brought much closer to the surface and appears more resistive. The differences between the two inversions are such that joint inversion of the TE and TM data is unable to fit both modes simultaneously, with the fit to the TM mode data being seriously degraded by the necessity to fit the TE data. It is inferred that the differences between the TE and TM inversions, and the failure of the joint inversion, reflect the effects on the TE mode data of the lack of continuity of structure along the regional strike direction and the 3D effect of Cook Strait. DISCUSSION The main features of the resistivity structure presented in Fig. 8 are consistent with the basement structure derived from modelling of gravity data by Hicks & Woodward (978). The deep region of low resistivity occurring adjacent to the Wairarapa Fault is consistent with the trough of Cenozoic sediments identified by the gravity data. The 3D gravity modelling of Hicks & Woodward (978), using an average density contrast of 0.4 Mg/m 3 between sediment and basement greywacke, placed a thickness of up to 3 km on these sediments. The smooth nature of the D resistivity inversion makes it difficult to identify actual resistivity boundaries, but the steep rise in resistivity, from 0 to >00 Ωm, which occurs between 3 and 4 km depth, is consistent with such a thickness. Hicks & Woodward (978) inferred a thickness of sediment of c. km in the eastern arm of the Wairarapa trough identified by the gravity data. This arm of the trough corresponds to the region between MAR and HIN in the resistivity model, where conductive sediments of resistivity 0 Ωm persist to c. km depth beyond which there is again a marked rise in resistivity. There is some indication from the resistivity model that the higher resistivity basement dips to the northwest, from the outcropping greywacke of the coastal ranges, at an angle of c. 0. This is consistent with the value obtained from seismic reflection by Cape (989) and Cape et al. (990) just to the south of the MT site MAR. The high-resistivity feature intruding between the two regions of sediment accumulation can be identified with the spur of Mesozoic basement found by Hicks & Woodward (978) to trend approximately north from the Aorangi Mountains. Outliers of exposed greywacke exist to the north of the MT transect (Fig. ), and such a feature is possibly related to the sharp rise in resistivity observed by Ingham et al. (00) at the southern end of their transect (AA ) along the Hikurangi margin. Henderson (999), on the basis of new gravity data, has proposed the existence of a faultbounded block of high-density material between the MT sites ONGA and MARA. This is interpreted to be related to the occurrence of basement greywacke at shallow depth. At very shallow depth, both the vertical and lateral resolution of the resistivity structure shown in Fig. 8 is poor, but recent dc resistivity soundings across this feature (McLoughlin 000) suggest that the resistivity starts to increase at only c. m depth. Higher frequency MT data with the use of continuous electric field profiling, as suggested by Fraser (00), may be able to provide a better electrical image of this faultbounded feature. A sharp lateral contrast in resistivity that extends to several kilometres depth and which, within the limits of resolution of the MT site spacing, appears to coincide with the Wairarapa Fault is evident in Fig. 8. The D gravity models of Hicks & Woodward (978) suggested that the Wairarapa Fault is reverse in this region. However, Bowler (998) presented a best fitting gravity model that showed it to be vertical. The high-resistivity contrast between the greywacke of the main axial ranges and the sediments of the Wairarapa trough indicates that more detailed MT data have the potential to be able to assist in resolving the dip of the fault (Fraser 00). The results of this study, taken in conjunction with those of Ingham et al. (00), support the interpretation that, although the electrical structure along the length of the Hikurangi margin from Hawke Bay to Cook Strait is relatively uniform, the structure is more complex in this southwestern part of the margin. Farther to the north, Ingham et al. (00) found conductive sediments to extend c. 50 km inland from the coast and to have a similar thickness to that inferred here. Interpretation of data from MT sites along the length of the margin showed uniformity along the margin until a rise in resistivity observed in the vicinity of the present transect. The model shown in Fig. 8 indicates that the overall lateral extent of conductive sediments is similar to that along the northern line, but is broken by the extension of shallow basement structure north from the Aorangi Mountains. A southward increase in the complexity of phase data observed by Ingham et al. (00) can be related to the existence of this and the proximity of one of the outliers of greywacke that occur to the north of the present transect (Fig. ). The extent of this feature to the north remains uncertain. The increased complexity of the structure at the southwest end of the Hikurangi margin, including the proximity to Cook Strait, makes it unlikely that an electrical signature associated with the subduction interface can be detected. ACKNOWLEDGMENTS This work was supported by New Zealand Foundation for Research Science and Technology Contract 96-GNS and the UK Natural Environmental Research Council (NERC) Grant GR9/ 3397, and also through equipment loans from the NERC Geophysical Equipment Pool. We thank Oliver Ritter for providing the robust processing software for the SPAM III data and Ute Weckmann for her help in using it. We also thank Daniel Pringle, Douglas Fraser, and Allister Richardson for assistance in the field, Eric Broughton for extensive technical help, and the many landowners on whose properties measurements were made. REFERENCES Beanland, S. 995: The North Island dextral fault belt, Hikurangi subduction margin, New Zealand. Unpublished PhD thesis, Victoria University of Wellington, Wellington, New Zealand. Bowler, S. J. 998: The gravity signature of the Wairarapa Fault in the Featherston region. Unpublished BSc Hons thesis, Victoria University of Wellington, Wellington, New Zealand. Cape, C. 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